Mesh Bisect Algorithm: How does it actually work?
As you might have heard, blender is open source software, which means the full source code is public. You can download the source code for example from blender's official download page. (Link on the same page above source download leads to info about setting up dev environment and such, if you have more interest on project)

For particular file inside the project, bisect tool is found under source / blender / bmesh / tools / bmesh_bisect_plane.c/h.